1. Field of the Invention
The invention is related to the field of wafer fabrication and, in particular, to testing the quality of lift-off processes used in wafer fabrication.
2. Statement of the Problem
Wafer fabrication is a procedure composed of many repeated sequential processes to produce electrical circuits, devices, chips, etc. For example, wafer fabrication is typically used to build semiconductor components, such as amplifiers or transistors. Wafer fabrication is also used to build magnetic recording or magnetic memory devices, such as magnetoresistance (MR) read elements, write elements, etc. The electrical circuits, devices, chips, etc, that are formed on a wafer are referred to herein as functional components.
Photolithography is often used in the wafer processes to pattern or shape thin films. Thin films may be patterned by either etching or lift-off processes. For the etching process, one or more thin-films are deposited on the wafer before the photoresists are defined. The photolithographic process is then performed to define the photoresists on top of the thin-films. The photoresists are patterned to protect the portions of the thin-films underneath the photoresists. An etching process is then performed to remove the areas of the thin-films that are not covered by the photoresists. The etching process may comprise a liquid or plasma chemical agent that removes the areas that are not protected by the photoresist. Etching can also be performed by physical bombardment with ions. After a photoresist is no longer needed, the photoresist may be removed. The photoresist may be removed with a liquid solution, or may be removed with a reactive ion etching process.
For the lift-off process, the photoresists are defined on the wafer before the thin films are deposited. The photolithographic process is performed to define the photoresists on top of the wafer. The photoresists are patterned to remove the portions of the thin-films that are subsequently deposited on top of the photoresists. One or more thin films are then deposited, which cover the photoresists and the exposed portions of the wafer. A lift-off process is then performed to remove the areas of the thin-films that are deposited on the photoresists. For the lift-off process, the photoresists are dissolved with a resist solvent. When the photoresists are removed, the areas of the thin-films that are deposited on the photoresists are washed away leaving holes in the thin-films.
One problem encountered in wafer fabrication is determining the quality of a lift-off process. If the lift-off process is poor, then the thin-films will not be removed in the areas where a hole is supposed to be. A poorly-formed hole may be caused for a number of reasons. For instance, the thin-films may be too thick, a photoresist may be too thin, or an undercut may be formed in the photoresist to avoid deposition fencing. In these scenarios, the thin-films may cover the top of the photoresist and the sides of the photoresist, and the lift-off solution will not be able to dissolve the photoresist wholly or even partially. If the photoresist cannot be dissolved, then the thin-films deposited on top of the photoresist will not be removed resulting in a poorly-formed hole.
Presently, when a lift-off process is performed on a wafer, the wafer is visually inspected with a microscope to determine if the holes in the thin-films were adequately formed. If one or more of the holes are poorly formed, then the lift-off process needs to be adjusted. This method of inspection may be inadequate especially when the areas of lift-off get smaller and the number of sites becomes numerous, because it is harder to visually determine whether the holes are open and adequately formed. It is therefore desirable to have alternative ways of determining the quality of a lift-off process in wafer fabrication.